Bank credit cards, employee identification (ID) badges, coded tags and the like frequently have a magnetic strip on a face of thereof Coded information such as an account status, employee ID number, entry authorization, etc., is recorded on the strip for subsequent retrieval and use. To read the information on the card, it must be physically placed in a suitable card reader apparatus. Only one card at a time can be read by a given card reader and it is necessary to bring each card directly to the card reader. While this way Of identification of a card has the advantage of very low cost coding of the card, it has the important disadvantage of requiring the physical insertion of the card into a card reader, and the need for queuing of the cards to be read in turn, one at a time.
Various systems for the remote identification of moving objects such as railroad cars, have been developed in the past. One of the simplest systems uses an optical bar code applied to the side of each car. As the car moves past a given location an electronic monitor projects a laser beam from some distance away and scans the bar code. The monitor picks up reflected light from the laser beam as the bar code is scanned and produces electronic signals corresponding to the code for that particular car. While such a system can read a coded object (via the bar code) on-the-fly from some feet away, the system becomes inaccurate or inoperable when fog, dirt, and the like obscure the reflected laser light. Even when working as intended, the system relies on the laser light beam impinging on and being properly reflected back from the optical bar code within certain narrow angles.
Use of radio frequency as the data communication link in remote interrogation/identification (I/I) systems is also well known. An article entitled "Microwave Tag Identification Systems" by Daniel D. Mawhinney, pp. 589 to 610, RCA Review, Vol. 44, December 1983, discusses in detail various kinds of such systems using microwave frequencies. As this article points out, coded objects (such as badges, tags, etc.) to be identified can be categorized as "passive", "driven", and "active". An example of a "passive" tag is one having an optical bar code. A simple example of a tag which is "driven" is one having a diode and tuned circuit which when energized by an interrogating radio frequency (RF) signal emits an uncoded response signal. An example of an "active" tag is one which has its own power source (typically a battery). When interrogated, an active tag responds by transmitting a coded signal which serves to uniquely identify the tag. Disadvantages of passive tags are the considerable difficulties of remotely interrogating and uniquely identifying each of them with RF signals. Similarly, driven tags have these same disadvantages to one degree or another, and further require that the interrogating RF signal provide a considerable amount of power to energize adequately the driven tag in its response. This requirement that the RF signal be powerful enough to energize a passive tag means that such a system is severely limited in its applications because of concern for human safety where high frequency radio field energy is present, and because of FCC radio transmission restrictions. "Active" tags on the other hand, have in the past been bulky and of limited operating life because of the relatively large amount of power consumed by the tag circuitry and the limited power capacity of a small battery. In the microwave system using active tags described in the above identified article, the tags themselves were relatively costly, and the number of code combinations which could be used were limited.
It is desirable for several reasons to employ a microwave beam (with beam power well below health and safety limits) to interrogate personnel ID badges and the like. A microwave signal has a relatively short wavelength and hence is easy to focus into a directional beam. Such a beam is easily able to pass through ordinary clothing and hence can detect and identify a badge even when carried in a person's pocket. Moreover, a directional beam is less likely to be confused by unwanted reflections, or other kinds of interference. Additionally, a very large amount of coded information for interrogating and uniquely identifying a badge or badges can be transmitted via a microwave signal in a very short time. But in order for electronically coded badges to be useable with a low power microwave beam, the electronic circuitry employed in a badge must have excellent input sensitivity at microwave frequencies, it should also, as a practical matter, be able to accommodate a very large number of code combinations (e.g., many millions). And in order to operate for a long time (many years) with a minute-size battery the circuit must draw extremely low average current (e.g., substantially less than 1 microampere). These combined objectives have not previously been met.
In an article entitled "Personnel-Tracking System", by McEachern, Prost, Hampel and Mawhinney, pp. 57 to 63, RCA Engineer, 28-6, Nov. - Dec. 1983, the authors describe a microwave-based tracking system in which a uniquely coded, battery powered badge (credential) is issued to each user. The badges are interrogated by a 10.5 GHz microwave carrier having sixteen frequency-modulated tones. Each badge is set to monitor one of these tones and, upon detection, transmits a return pulse in one of sixty-four time slots on one of eight preassigned frequencies in the range of 200 to 300 MHz. A number of badges in a given location can thus be interrogated all together and identified one by one separately. However, this system permitted only about 8000 code combinations and the battery in a badge lasted only about 3 months due to heavy current drain by the electronic circuit of the badge.
A microwave identification system described in U.S. Pat. No. 4,912,471 has a coded "target" (electronically coded device) which is powered separately (though momentarily) by an auxiliary low frequency energizing field (e.g., 25 KHz). The target, using internally stored electric power derived from the auxiliary energizing field, then transmits its coded information to an interrogator when the interrogator scans the target with an unmodulated microwave beam (915 MHz). The target modulates the impedance of its receiving antenna in accordance with coded information stored in memory within the target. In this way a reflected, code-modulated microwave signal from the target antenna is sent to the interrogator where the signal is received and decoded. There is no transmission of data (other than the unmodulated microwave beam itself) from the interrogator to the target. The microwave beam used in this system has a low power level (within FCC regulations), and a given target is able to accommodate a large number of code combinations. But an important restriction on use of this system is that it is able to interrogate and identify only one target at a time. The system is unable to handle multiple targets all at the same time. Moreover, the system is relatively complex and costly.
It is desirable to provide an electronic I/I system which overcomes most, if not all, of the limitations, restrictions and drawbacks of the systems described above. To this end coded articles such as badges, tags and the like should have ultra-low power consumption to operate with a micro-size battery for a time substantially as long as the shelf-life of the battery (e.g., 4 years or more), very low cost, excellent sensitivity at suitable frequencies (e.g., microwave) to provide reliable operation with RF field power levels well below the limits permitted by FCC regulations and health and safety standards, ability to handle a very large number of code combinations, ultra-secure code storage along with the ability to remotely re-write the code, ability for a number of different articles to be remotely interrogated at the same time (a small fraction of a second) and for each article to be uniquely identified while still in the presence of the others, and small size.
It is also desirable that an interrogator/reader (I/R) unit for use with the articles use low cost off-the-shelf components for radio (microwave) transmission and for data coding and decoding, have the ability to transmit and receive very low power encoded signals at radio frequencies (no site licensing requirement, no health hazard), the ability to operate with a unique algorithm to interrogate/identify multiple articles simultaneously (in a small fraction of a second), input/output (I/O) interface with various code formats, such as RS-232, Weigand, Track 2ABA, etc and the ability to remotely program the articles with new identification data.